Crack detecting window panel and method of producing same

ABSTRACT

A heated window assembly for a vehicle includes a pair of spaced apart bus bars connected by a conductive film. A sense line is connected to one of the bus bars to monitor and compare the voltage applied to the bus bar. A control circuit connected to the sense line detects a difference in voltage in the bus bar as compared to a reference voltage and disconnects the bus bars from the vehicle power supply.

BACKGROUND OF THE INVENTION

The present invention concerns a crack detecting window panel assemblyand method for producing same, and in particular, a method formonitoring and interrupting power applied to an electrically heatedwindow glass.

Motor vehicles of recent manufacture are being equipped withelectrically heated window panel assemblies, i.e., windshields, fordefogging and deicing the windows. These electrically heated windshieldsare becoming increasingly popular, not only due to the convenience theyafford the driver with rapid deicing and defogging, but also from thethe enhanced safety they afford in the vehicle's operation. Theseelectrically conductive windshields typically draw 5 to 35 amperes ofelectric current at 12 to 100 volts during heating depending upon themethod and materials used in their construction. With these high levelsof electrical current consumption, potential problems due to fracturesin the window exist.

One possible problem which arises is the potential for electric shock toan individual coming into contact with a fractured windshield andreceiving an electrical shock due to power still being applied to theelectrically conductive panel.

Another problem is the possibility of local ignition and flame due to anarcing which may occur when a fracture exists in the electricallyconductive panel.

Yet another possible problem is the potential for a motor vehicleaccident due to startling the driver with the electric arc from thefractured window and then the driver losing control of the vehicle.

Prior artisans have developed crack detectors which have concentrated ontwo avenues of methodology for circuit interruption. These methods haveinvolved resistance level detection or continuity loops to indicate whena crack has formed in the windshield. An example of one of theresistance measuring type of systems is shown in U.S. Pat. No.4,565,919, in which the circuit includes structure for monitoring theresistance of the conductive element and interrupting the power appliedthereto when the resistance of the conductive element assumes a valueindicative of the element being fractured. In another example of thetype in which a continuity loop is used, as shown in U.S. Pat. No.3,941,975, a sensing circuit which includes a frangible strip isdisposed in a predetermined pattern on a panel. When the panelfractures, one or more cracks propagate across the frangible sensingcircuit to interrupt the same for deenergizing the electricallyconductive panel. Finally, U.S. Pat. No. 3,892,947 discloses anelectrically heated, frangible panel especially adapted to preventelectrical shocks after fracture or breakage. An electric currentconductive strip is adhered in the marginal band and extends around anelectrically conductive coating before contacting an edge thereof. Uponfracture or breakage of any portion of the panel, the conductive stripsevers preventing current flow through the coating.

SUMMARY OF THE INVENTION

The present invention concerns a novel method for interrupting the powerto an electrically conductive panel upon detection of a crack in same.Essentially, a voltage sensing circuit is provided for the electricallyconductive panel which consists of a voltage sense line and bus barswhich are silk screened onto a major surface of a glass sheet or onto aceramic enamel band applied to the major surface of the glass sheeteither prior to or subsequent to the application of an electricallyconductive film. The bus bars and the voltage sense line areinter-connected during a silk screening process, and are typicallyformed of a silver frit material. The glass sheet is heated to fire thevoltage sense line and bus bars into place. The electrically conductivefilm is applied to the same surface of the glass panel. The electricallyconductive film is applied to allow contact with the bus bars to form acomplete electrical circuit. The filmed panel is typically assembled toan intermediate plastic sheet and ultimately to a second glass panel toform a laminated window assembly.

The crack detecting window assembly in accordance with the presentinvention interrupts power upon detection of a fracture of said windowassembly by sensing differential voltage levels between a selected busbar and the sense line. Upon detection of a fracture of the selected busbar or the sense line, the voltage of either the bus bar, or the senseline will deviate from the norm, and the electrical circuit will beinterrupted to prevent the problems usually associated with fracturedelectrically conductive windows. The interruption of the power isaccomplished via a voltage sensing circuit provided in the motor vehicleelectrical system. The sense line is connected to an upper bus bar nearthe side edge of the window assembly, which is the area where crack shutdown is needed due to the high current level being carried by the busbar. The present invention has the advantage of requiring only threeelectrical connections to the window assembly whereas most prior artdevices require four connections.

BRIEF DESCRIPTION OF THE DRAWINGS

The above, as well as other advantages of the present invention, willbecome readily apparent to those skilled in the art from the followingdetailed description of a preferred embodiment thereof when consideredin the light of the accompanying drawings in which:

FIG. 1 is a perspective view of a windshield assembly according to thepresent invention;

FIG. 2 is an enlarged cross-sectional view taken along the line 2--2 ofFIG. 1;

FIG. 3 is an enlarged fragmentary plan view of a portion of thewindshield assembly shown in FIG. 1; and

FIG. 4 is a flow diagram of a method of producing an electrically heatedwindshield assembly with a voltage sensing circuit according to thepresent invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIGS. 1--3 show an electrically conductive windshield in accordance withthe present invention. The windshield, indicated generally at 10,includes an outboard sheet of glass 12, an intermediate sheet of plasticmaterial 14, for example composed of a polyvinyl butyral or polyestermaterial, and an inboard sheet of glass 16 all laminated together underheat and pressure to produce an integral unit. For purposes ofreference, the surfaces of the glass sheets 12 and 16 are typicallydesignated as first through fourth surfaces. Thus, an outer surface 18of the outboard glass sheet 12 is the number one or first surface. Aninner surface 20 of the outboard glass sheet 12 is the number two orsecond surface; an inner surface 22 of the inboard glass sheet 16 is thenumber three or third surface; and an outer surface 24 of the inboardglass sheet 16 is the number four or fourth surface.

Typical of the structure of electrically heated windshields used incurrent motor vehicles, a film of electrically conductive materialextends between a pair of spaced bus bars located along opposed edges ofa windshield. From the aesthetic standpoint, it is desirable to shieldor hide the bus bars from view outside the windshield. Typically, thisis accomplished by applying a peripheral band 26 of opaque ceramicenamel material, usually through a silk screening process, to the secondsurface 20. After the band 26 has been applied and dried, a pair of busbars are applied on top of the band. An upper bus bar 28 extends acrossthe upper periphery of the windshield assembly 10, down one side of thewindshield and along a portion of the bottom of the windshield beforeterminating in an electrical lead connector 30. A lower bus bar 32extends across the lower periphery of the windshield assembly 10 on theband 26 and has an electrical connector section 34 formed therewith. Atthe same time that the bus bars are applied in place, a sense line 36 isalso applied outboard of and adjacent to the upper bus bar 28. The senseline 36 is connected to the upper bus bar 28 at a predeterminedlocation, for example, at the right side of the upper periphery near thefront or "A" pillar of a vehicle as illustrated at 37 in FIG. 1. Thelocation at which the sense line 36 attaches to the bus bar willdetermine the amount of area in which a crack can be detected in theelectrically conductive windshield 10 and the location can be moved inorder to detect larger or smaller areas. The sense line 36 extends alongthe bottom portion of the windshield before terminating in electricallead connector section 28. The bus bars 28 and 32, and the sense line 36can be formed of the same silver frit material which is typically silkscreened in the desired pattern in a single process operation eitheronto the glass sheet or onto the ceramic band 26. This outboard glasssheet subassembly is then heated to fire the bus bars and sense line andalso to complete the fusing of the band 26 if this is not accomplishedin the drying step. At the same time, the sheet can be bent to thedesired shape for the windshield utilizing press or gravity bending.

After the outboard glass sheet has been shaped, an electricallyconductive film 40 is applied to the second surface 20. Typically, thefilm 40 is formed by sputtering techniques and comprisesdielectric-metal-dielectric layers, having optical properties suitablefor heated windshields as is well known in the art. As examples of suchfilms and methods of applying same, reference is made to European PatentApplication No. 0 219 273, published Apr. 22, 1987. The film 40 isconnected, e.g., overlaps, along its upper edge to the upper bus bar 28and is connected along its lower edge to the lower bus bar 32. As shownin FIG. 3, the edges of the conductive film can extend onto the band 26,but must not contact that portion of the upper bus bar 28 which runsdown the side of the windshield assembly, nor can it contact any portionof the sense line 36 in order to avoid establishing an unwantedelectrical circuit. Thus, the portions of the bus bar 28 along the sideperiphery and the lower periphery of the glass sheet 12, along with theentire sense line 36 must be masked prior to the applications of theconductive film 40 with a masking material 42 such as a tape or spraycoating for example, or any other masking material which prevents thefilm 40 from being deposited during the coating process (FIG. 3). Themasking material 42, shown outlined in phantom line, can be removedafter the film 40 is applied, or left in place to become part of thewindow assembly 10. Three electrical leads 44, 46 and 48 can be attachedto the connector sections 30, 34 and 38 respectively for connection tothe vehicle's electrical and crack detection systems upon installationof the windshield assembly. In order to expose the connector sections30, 34 and 38 and provide for good solder strength, the conductive film40 and the mask 42 must not cover the sections 30, 34, 38 and a cutoutportion 49 of the glass sheet 16 and intermediate sheet 14 must beremoved as shown in FIG. 3.

The method according to the present invention for producing a crackdetecting windshield assembly 10 is shown in flow diagram form in FIG.4. The process starts as illustrated at 50 and enters a first step 52wherein the enamel band 26 is applied to the number two surface of theouter flat sheet 12 of the window assembly. The next step 54 in theprocess is to heat the outer sheet 12 by exposing same to a temperaturenot exceeding 500° F. for from two to eight minutes to dry the band, orthe temperature may be elevated to a range of 900° F. to 1250° F. toprefire the band 26. A third step 56 in the process is to apply the busbars 28 and 32 and the sense line 36 to the enamel band 26 after theouter sheet 12 has cooled sufficiently. Typically, the bus bars andsense line are formed of a silver material, for example "Silver Paste",obtained from E.I. duPont De Nemours and Co. under the trade designation"No. 9999", which is silk screened onto the enamel band 26.

A fourth step 58 in the process is to heat the outer sheet 12 bysubjecting it to a temperature in the range of 900° F. to 1250° F. for aperiod of two to eight minutes to fire the bus bars 28 and 32 and thesense line 36, complete, if necessary, the firing of the enamel band 26,and soften the sheet for bending. While the outer sheet 12 is heated, afifth step 60 is performed wherein the sheet is bent to the desiredshape for the final windshield assembly 10. A sixth step 62 in theprocess is to mask portions of the bus bar 28 and the entire sense line36 with the masking material 42 to prevent an unwanted electricalcircuit in the final window assembly 10. A seventh step 64 is performedbetween steps 62 and 66 whereby a washing operation removes anymaterials emitted during the prefiring step 54 and the firing step 58.An eight step 66 is to apply the conductive film 40 to the number twosurface of the outer sheet. At this point the mask 42 coated with thefilm 40 can be removed if desired. A tenth step 70 in the process is toassemble and laminate the outer sheet 12 to an inner sheet of glass 16which has been bent to the desired shape i.e., to match the outer sheet.Typically, the lamination is achieved utilizing an intermediate sheet ofplastic material, i.e., polyvinyl butyral. The process is thencompleted, as illustrated at circle 74.

In order to expose the connector sections 30, 34 and 38 for subsequentsoldering to the ends of the electrical leads 44, 46 and 48, a sectionof the inner glass sheet 16 and any intermediate plastic sheet must becut away. Thus, a ninth step 68 is shown to indicate that the operationsof forming the cutout 49 and for soldering can be performed betweensteps 66 and 70. It should be noted that the ninth step 68 can beeliminated through the use of a previously notched inner glass sheet 16and a previously notched intermediate layer 14. Also shown in FIG. 4 isan eleventh step 72 wherein electrical leads are attached to the busbars and sense line. These electrical leads interface with a vehiclecircuit 74 (shown in FIG. 3) for providing power to the electricallyconductive windshield, and in particular to the present invention, andfor monitoring voltage and interrupting power upon detection of afracture in the windshield.

In operation, the windshield assembly 10 is supplied with electric powerto the bus bars 28 and 32 on the leads 44 and 46 by the vehicle powerand control circuit 76. Typically, the vehicle operator positions aswitch in the vehicle in the on position thereby applying voltage to thebus bars 28 and 32 and current flows in the conductive film 40 in thewindshield assembly 10. As the current flows in the windshield assembly10 through the bus bars 28 and 32, the conductive film 40 heats andtransfers the heat energy by the conductive process through thecomponents making up the windshield assembly 10 thereby deicing anddefogging the window assembly.

The vehicle circuit 76 includes a power unit 78, a power switch 80 and acontrol unit 82. The power unit receives power, either A.C. or D.C.,from a vehicle power supply 84 such as a battery and an alternator. Ifthe heated window is to be excited with a voltage higher than thatavailable from the vehicle power supply 84, the power unit 78 steps upthe magnitude of the voltage by any conventional device which can alsoinclude current and voltage limiting means. The electrical power fromthe power unit 78 is applied to the leads 44 and 46 through the powerswitch 80. A vehicle operator switch 86, typically located within reachof the driver, is connected to the power switch 80 to manually controlthe application of power to the window assembly 10. The control unit 82is connected to the lead 48 to receive the sense line voltage and isconnected to the power switch 80 input by a line 88 to receive the powerunit output voltage as a reference. When the voltage on the sense line36 falls more than a predetermined amount below the magnitude of thepower unit voltage, the control unit 82 signals the power switch 80 todisconnect the power from the leads 44 and 46.

During the heating operation of the window assembly, the vehicle powerand control circuit 76 is constantly monitoring and comparing themagnitude of voltage being supplied to the upper bus bar 28 and thesense line 36 with the reference voltage value. Upon the formation of acrack in the upper bus bar 28 or the sense line 36, between the location37 and the leads 44 or 48, the unit 76 senses a voltage difference andinterrupts the power being supplied to the window assembly 10. Theformation of a crack in the window assembly severs the upper bus bar 28or the sense line 36 causing the voltage on the line 36 to drop. Thevehicle circuit 76 detects a differential in the voltage being suppliedto the severed bus bar 28 or the sense line 36 and immediatelyinterrupts the power to the window assembly 10. Prior systems for crackdetection have allowed a period of time before the power to theelectrically conductive panel would be interrupted upon formation of afracture, thereby allowing arcing across the break, and possible healingof the break due to a welding effect. In accordance with the presentinvention, the above described system instantaneously interrupts thepower being supplied to the panel, which provides additional safety andprotection for the vehicle operator.

The vehicle circuit 76 is normally supplied by the vehicle manufacturer,however, any commercially available circuit for monitoring and sensingdifferential voltage can be used with the present invention.

In an alternative embodiment from that described above, the conductivecoating can be applied prior to the shaping process of the windowassembly. The use of a pyrolitic conductive coating for example, allowsfor bending of conductive coating along with the glass sheet, therebyallowing the coating to be applied to the glass sheet while still flatand prior to bending.

In another embodiment, the conductive film can be applied to alternativesurfaces of the conductive panel. For example, the conductive film maybe applied to one of the surfaces of the plastic interlayer, on aplastic sheet forming an anti-lacerative shield or laceration shieldsurface, see for example U.S. Pat. Nos. 4,107,366 or 4,242,403, or onany number of surfaces within the window assembly. If the conductivefilm is to be applied to a plastic sheet rather than a glass sheet, thenthe bus bars 28 and 32, and the sense line 36 normally would be formedfrom an electrically conducting polyester material for example. Thepolyester material bus bars and sense line would then be attached to theplastic layer by gluing or the like prior to the application of theconductive film on the plastic layer. The plastic layer can then beadhered to a glass sheet, another plastic sheet or any other suitabletransparent material, thereby allowing for a crack detecting lacerationshield type panel, which can be a bi-layer or any other conventionalwindow panel.

In accordance with the provisions of the patent statutes, the presentinvention has been described in what is considered to represent itspreferred embodiment. However, it should be noted that the invention canbe practiced otherwise than as specifically illustrated and describedwithout departing from its spirit or scope.

What is claimed is:
 1. A window assembly adapted to be electricallyheated, comprising:(a) a first transparent sheet having a major surface;(b) a pair of spaced apart bus bars overlaid on and adhered to saidmajor surface of said first sheet and each having a connector sectionadapted to be connected to an electrical power supply; (c) a transparentelectrically conductive film adhered to said major surface andelectrically connected between said bus bars; (d) at least one senseline overlaid on and adhered to said major surface, said sense linebeing connected to one of said bus bars at a predetermined locationspaced from said connector section of said one bus bar for sensing avoltage applied to said one bus bar and said sense line adapted to beconnected to a device for controlling an electrical power supplyconnected to said bus bars; and (e) a second transparent sheet adheredto said first transparent sheet.
 2. A window assembly adapted to beelectrically heated, comprising:(a) a first transparent sheet having amajor surface; (b) a pair of spaced apart bus bars overlaid on andadhered to said major surface of said first sheet and adapted to beconnected to an electrical power supply; (c) a transparent electricallyconductive film adhered to said major surface and electrically connectedbetween said bus bars; (d) a sense line overlaid on and adhered to saidmajor surface, one end of said sense line being connected to one of saidbus bars intermediate opposite ends of said one bus bar for sensing avoltage applied to said one bus bar and an opposite end of said senseline adapted to be connected to a device for controlling an electricalpower supply connected to said bus bars; (e) an intermediate layer of anadhesive material adhered over said major surface of said first sheetand said film; and (f) a second transparent sheet adhered to saidintermediate layer.
 3. A window assembly according to claim 2, whereinsaid adhesive material is a preformed plastic layer.
 4. A windowassembly adapted to be electrically heated, comprising:(a) a first outertransparent sheet having an inner major surface; (b) a pair of spacedapart bus bars overlaid on and adhered to said major surface of saidfirst sheet and each having a connector section adapted to be connectedto an electrical power supply; (c) a transparent electrically conductivefilm adhered to said major surface and electrically connected betweensaid bus bars; (d) at least one sense line overlaid on and adhered tosaid major surface, said sense line being connected to one of said busbars at a predetermined location spaced from said connector section ofsaid one bus bar for sensing a voltage applied to said one bus bar andsaid sense line adapted to be connected to a device for controlling anelectrical power supply connected to said bus bars; (e) an intermediatelayer of plastic material adhered over said major surface of said firstsheet and said film; and (f) a second inner transparent sheet adhered tosaid intermediate layer.
 5. The window assembly according to claim 4wherein said first and second sheets are formed of a glass material. 6.The window assembly according to claim 4 wherein said intermediate layeris formed of a polyvinyl butyral material.
 7. The window assemblyaccording to claim 4 wherein said intermediate layer is formed of apolyester material.
 8. The window assembly according to claim 4 whereinsaid bus bars and said sense line are formed of a silver material. 9.The window assembly according to claim 4 wherein a portion of said onebus bar extends along an upper edge of said first sheet and a portion ofanother of said pair of bus bars extends along a lower edge of saidfirst sheet, and said film is connected between said bus bar portions.10. The window assembly according to claim 4 including a band of opaquematerial adhered to said major surface and wherein said bus bars andsaid sense line are adhered to said band.
 11. The window assemblyaccording to claim 10 wherein said band is formed of a ceramic enamelmaterial.
 12. The window assembly according to claim 4 wherein aninsulating masking material is located between a portion of said one busbar and said film.
 13. The window assembly according to claim 4 whereinan insulating masking material is located between said sense line andsaid film.
 14. The window assembly according to claim 4 wherein aportion of each of said film, said intermediate layer and said secondsheet are cut away to expose said connector section of each of said busbars and a connector section of said sense line.
 15. The window assemblyaccording to claim 14 including an electrical lead connected to each ofsaid connector selections of said bus bars and said sense line.
 16. Avehicle window assembly adapted to be electrically heated from a vehiclepower supply, comprising:(a) an outer glass sheet having an inner majorsurface; (b) an opaque band adhered to a peripheral portion of saidmajor surface; (c) an upper bus bar adhered to said band and extendingfrom a lower edge of said outer sheet along a side edge and an upperedge of said outer sheet; (d) a lower bus bar adhered to said band andextending along said lower edge of said outer sheet; (e) a sense lineadhered to said band and extending generally parallel to said upper busbar, one end of said sense line being electrically connected to saidupper bus bar between opposite ends of said upper bus bar; (f) anelectrically condcutive film adhered to said major surface over saidband, said bus bars, and said sense line; (g) an intermediate layer ofplastic material adhered to said film; (h) an inner glass sheet adheredto said plastic material layer; and (i) three electrical leads eachconnected to an associated one of said bus bars and said sense lineadjacent said lower edge of said outer sheet.
 17. The window assemblyaccording to claim 16 wherein a portion of each of said film, saidintermediate layer and said inner glass sheet are cut away to exposesaid connections of said leads.
 18. The window assembly according toclaim 16 including a power switch connected between a vehicle powersupply and a pair of said leads connected to said bus bars.
 19. Thewindow assembly according to claim 18 including control means connectedbetween a one of said leads connected to said sense line and said powerswitch for actuating said power switch to disconnect said bus bars fromsaid vehicle power supply in response to a predetermined voltage drop onsaid sense line.
 20. The window assembly according to claim 16 includingan insulating mask positioned over said sense line and over a portion ofsaid upper bus bar adjacent said side edge and said lower edge of saidouter sheet.
 21. A method of manufacturing an electrically heated windowassembly comprising the steps of:(a) providing an outer transparentsheet having an inner major surface; (b) applying an opaque ceramicenamel material in a band around a periphery of said major surface; (c)adhering a pair of bus bars and a sense line to said band, a lower oneof said bus bars formed along a lower edge of said outer sheet, an upperone of said bus bars formed along an upper edge and one side edge ofsaid outer sheet, and said sense line formed between said upper bus barand said side edge; (d) applying an insulating mask over said sense lineand over a portion of said upper bus bar adjacent said sense line; (e)applying an electrically conducting film over said major surface, saidband, and said bus bars; (f) adhering an intermediate plastic layer tosaid film; and (g) adhering an inner transparent sheet to saidintermediate layer.
 22. The method according to claim 21 including astep of forming a cutout in said inner sheet, said intermediate layerand said film to expose a connector section of each of said bus bars andsaid sense line.
 23. The method according to claim 22 including a stepof attaching a separate electrical lead to each of said connectorsection.
 24. The method according to claim 21 including performing astep of removing said insulating mask between said steps (e) and (f).